Electric motors are well known and widely used. They come in a variety of sizes and styles. One example use of an electric motor is in an elevator machine that moves a drive sheave for propelling an elevator cab up or down through a hoistway, for example. Another use for an electric motor in a heating ventilation, air conditioning or refrigeration systems (HVACR).
Recently, regenerative drive machines have been introduced particularly into elevator systems. Regenerative drive machines include an electric motor that draws power from a power source for purposes of moving a car and counterweight through a hoistway in a first direction and generates power that is provided back to the power source when allowing the car and counterweight to move in an opposite direction. The regenerative drives take advantage of the ability of an electric motor to act as a generator when the weight of the car and counterweight cause the desired movement as long as the drive machine allows the drive sheave to be moved accordingly. Such regenerative drive machines typically operate on a three phase power input.
Active front end (AFE) converters in drives employ a pulse width modulated (PWM) switching rectifier to convert input AC power and provide DC power to a bus. Furthermore, inverter switching devices then convert the voltage DC bus to AC output currents to drive the load, e.g., motor. Such active front end converters are typically coupled with input filters, such as LCL filter circuits connected to each power phase. Since the front end rectifier is a switching circuit, the input filter operates to prevent introduction of unwanted harmonic content into the power grid. Filter components, including the filter inductors, and the converter switching devices are typically designed according to the power converter rating. Oversizing input filter components and switching devices adds cost to the system and occupies valuable enclosure space. In certain applications, it may be desirable to operate a higher voltage motor or other load even though the source voltage is low, for instance, a 400 V input voltage to drive a 460 V motor. In these situations, the active front end rectifier can be operated in boost mode to provide additional boost to increase the gain of the front end converter, thereby boosting the DC bus voltage. Likewise, under certain conditions it may be desirable to operate the inverter and motor at conditions where the current drawn from the DC bus is beyond rating, or the currents in the switching devices are beyond ratings. Active front and rectifiers/converters may also exhibit increased switching loss associated with operation of the converter switching devices under such conditions. Moreover, operation of an active front end power conversion system in boost mode may require an overall derating of the input and output capabilities of the converter. Specifically, the maximum output current available from the power converter may need to be reduced when the active front end is operated in boost mode in order to mitigate or avoid overheating the filter inductors and/or to reduce rectifier switching losses. However, such derating is inherently in efficient and may render a power conversion system unsuitable or not cost effective for a given application. Accordingly, there is a need for improved power converter apparatus and operating techniques to facilitate operation with an active front end while mitigating or avoiding thermal stress to filter inductors and/or rectifier switching losses to achieve improved power ratings.